Heparin, a parenterally administered thrombin inhibitor, is an effective coagulant, but its use can be limited by heparin-induced thrombocytopenia, a side-effect observed in about 5% of patients. In addition, heparin has limited efficacy in prevention of deep-vein thrombosis in total hip replacement. This proposal describes a novel approach for the isolation of an alternative thrombin inhibitor. The approach is based on a repetitive cycle of in vitro binding and PCR amplification which has yielded single-stranded DNA molecules with greatly enhanced affinity for thrombin compared to the unselected starting DNA. This DNA was cloned and isolates identified which inhibit thrombin activity. Inhibitory clones were sequenced and a highly conserved region, spanning about 15 bases, was identified. Our goal is to modify oligonucleotides to produce a therapeutic agent with potent anti-clotting activity.